Above you can see the circuit diagram for this circuit. As you can see the 555 timer sits central in the circuit. Pins 8 and 4 are connected to the positive side of the battery, to provide power and to stop the chip resetting. Pin 1 is connected to ground; this is to provide a potential difference across the 555 timer so it can operate. Pins 2, 6, and 7 are connected to the resistors and capacitors; these are used to control the speed at which the timer oscillates. The row of resistors are the different ‘keys’ on the keyboard, providing the different charging time for the capacitor and so changing the speed of oscillation. You can see the capacitor going directly from power to ground also; it is smoothing out the voltage. Finally you can see the speaker going from positive voltage to pin 3 of the 555 timer, providing the ‘note’ which we hear.

Overview/How it works:

This circuit uses a 555 timer integrated circuit to provide a note, which is then outputted by the speaker (or headphone jack). An integrated circuit is like a normal circuit, but miniaturised and entirely encased inside the grey plastic case, with only the necessary pins sticking out of it. In this circuit we use what is known as a 555 timer. This is an integrated circuit often used in timing circuits, you don’t need to know how this circuit works, but a basic knowledge of what is does helps in understanding this circuit. The 555 timer has 8 pins:

Pin 1 is the ground pin; this is the pin you connect to the negative side of your circuit.

Pin 2 is one of the control pins; it is used to control the operation of the 555 timer.

Pin 3 is the output pin; it is the pin which provides the ‘note’. It provides what is called ‘an oscillation’, this means it swaps between outputting a high voltage (in our case 9V) and outputting 0V. It does this at a certain speed (which is set by pins 2, 6 & 7). We use this to create a certain note on our speaker.

Pin 4 is the reset pin, when this pin is connected to ground the chip resets, so we keep this pin permanently attached to positive voltage.

Pin 5 is not connected to anything in our circuit.

Pin 6 is used along with pins 7 and 2 to control the 555 timers operation.

Pin 7 is also used to control the operation of the timer.

Pin 8 is the positive pin of the chip; this is where we supply positive voltage for the chip to run. It can use any voltage from 3V to 15V.

Speakers work using an electro-magnet and a diaphragm, the electro-magnet is connected to the diaphragm and every time it is activated the diaphragm moves down, when the electro-magnet isn’t powered the diaphragm moves back up again. This pushes sound waves out of the speaker so we can hear them. To do this we need an oscillation (like we mentioned before), but unlike what you would expect, we don’t use the output from the 555 timer directly, because it doesn’t supply enough power to move the speaker effectively. Instead we use the power line, and use the 555 timer as a ground pin.

Electricity needs a difference in voltage to flow (that’s why you have a positive and a ground) so if you apply the same voltage to either side of a component it will not work, you need a voltage drop (9V to 0V in this circuit) for the component to work. We call this a ‘potential difference’.

So when the 555 timer is outputting 9V the speaker will not move the diaphragm, however when it is outputting 0V it will, because of the potential difference between the 9V and the 0V. In this way we can use the power line directly to get more power into the speaker.

We have also used a capacitor in this circuit, capacitors are like miniature batteries; they store electricity and release it again when it is needed. They are used to set the speed of the timer in our circuit. The time they take to charge is dependent on the resistance before them, if the resistor is bigger then the capacitor will take longer to charge (because less current can get to it) and so the timer will oscillate slower. However, if the resistance is lower the capacitor charges quickly and the timer increases speed. In our circuit we change the resistance before the capacitor to change the speed of oscillation (and so the note).

Capacitors can also be used for smoothing out fluctuations in voltage, we use another capacitor to do this in our circuit. It is connected directly from power to ground.

Our project is shown here built on breadboard, we experimented with creating a printed circuit board as a prototype. We gave one of these to Erin to put together.

Start by placing the 555 timer in the middle of the breadboard, at the left side. Make sure the little ‘notch’ in the 555 timer is facing left!

Then place some hook-up wires into the breadboard as above. One wire should connect pins 2 and 6 together, and another should connect pins 4 and 8 together. Pin 1 should be wired to the blue line at the bottom, and pin 4 should be wired to the red line at the bottom. Pin 3 should have a wire coming off to a row much further right than the 555 timer (preferably 7 rows from the end of the 555 timer).

Now place in some more hook-up wire as above. The wires should jump two rows and alternate their vertical placement between top and second to top, except for the second to last one which should be on the middle row.

Next add the two capacitors, one of them should go between the red line and the blue line (with the short leg going to the blue line) and one should go between the blue line and pins 6 and 2 of the 555 timer (again the blue line should go to ground.

Next add in the resistors, they go in order of the circuit diagram, but reversed. So the 270W resistor is far left and the 1.5KW resistor and the 180W resistor are far right. They must be placed at a junction between two black wires, except the two cases in which there are two resistors one after another (in series) in which case the first one goes at a junction and the second one goes from the output of the first. There is also a 1KW resistor between pins 2 and 7 across the 555 timer.

Next add in your battery connector and battery (with the red wire going to the red line and the black wire going to the blue line) and also the longer wire from the red line which we will use to ‘play’ the circuit.

Finally add in some wires with the crocodile clips on the end between pin 3 of the 555 timer and the red line of the breadboard.

You can attach these to either end of a speaker (as shown here) if you are using a speaker.

Or if you are using a pair of headphones attach them to these parts of the headphone jack!

Now touch the exposed wire to one of the resistor leads and you should here a note! Moving the wire further left should create a higher note, and right a lower one. This circuit creates notes roughly a semitone apart, if you wish to tune these properly use a small variable resistor in place of the resistors and a tuner to tune the notes properly!

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